C++写法

#include <cstring>
#include <iostream>
using namespace std;

int main() {
    string username, password;
    while (cin >> username >> password)
    {
        if (username == "admin" && password == "admin")
            cout << "Login Success!";
        else
            cout << "Login Fail!";
    }
   
         
}

C的写法

#include <string.h>
#include <stdio.h>


int main() {
   
    char username[6], password[6];
    while (scanf("%s %s", username, password) != EOF)
    {
        if (strcmp(username, "admin") == 0 && strcmp(password, "admin") == 0)
            cout << "Login Success!";
        else
            cout << "Login Fail!"; 
    } 
         
}

注意使用strcmp函数两个字符串相等的时候返回的是0。